Add RavenDB support as a separate package

Hey guys,

I really like MiniProfiler and I also have been using Raven lately. The two plugins available, the official and the MiniProfiler one both didn't suit my needs. I really wanted to tie into the MiniProfiler the same way SQL is.

I found the Raven.Contrib project but their code was out-of-date and I figured it would be better as part of an official package.

Let me know what you think, any changes I should make, or whether you feel comfortable adding Raven to MiniProfiler officially. After making the required code changes, it's probably very possible to enhance the contrib project instead and keep it separated out, especially if I can add custom timings so easily now.